The present invention relates to compositions used to fabricate light emitting devices, more specifically, the present invention is drawn to organic-inorganic hybrid light emitting devices (HLED) fabricating with light emitting compositions in which hole transport, electron transport, and emissive compounds are covalently linked to polyhedral silsesquioxanes.
Flat panel displays based on organic light emitting devices (OLEDs) have been studied extensively by hundreds of industrial and academic institutions worldwide for the last decade. OLEDs offer exceptional potential for thinner, more energy efficient displays with equal or better resolution and brightness compared to the current liquid crystal display (LCD) technology. OLEDs also offer high switching speeds, excellent viewing angles ( greater than 160xc2x0), red, green and blue (RGB) color selection possibilities, and because no backlighting is necessary, it may be possible to fabricate devices on flexible substrates. However, despite the enormous research and development effort on OLEDs, there is presently only one commercially available product using this technology. One of the apparent problems for this is the need for the development novel efficient materials to satisfy the electronic device requirements.
The scientific basis for OLEDs relies on an organic material""s ability to emit light when subjected to electrical stimuli. In this process, electrons and holes are injected into organic materials from conducting electrodes and diffuse through a thin organic film to form electron-hole pairs or excitons within a highly conjugated organic molecule or polymer layer. The excitons then recombine creating an excited state within the organic molecule. The excited state can then undergo radiative decay emitting a photon. Depending on the organic polymer/molecule and its substituents, the wavelength of light emitted can be any color and even multicolored, e.g. red, green, blue or combinations thereof.
For optimal operation, it is important that the rate at which the holes and electrons diffuse into this emitting layer be similar, and preferably matched. Hence, numerous efforts have been made to optimize transport of both holes and electrons to the emitting layer and also to prevent trapping of holes or electrons that leads to destructive effects within the devices. Most recently, efforts have been made to incorporate organic molecules or polymers that promote movement of holes or electrons within the OLED device. Still more recently, efforts have been made to incorporate organic molecules or monomer units in polymeric systems such that one organic unit promotes hole or electron conduction and a second organic unit promotes emission. Such electronic tuning is designed to minimize the transport distances and maximize the hole/electron injection balance, thereby enhancing the potential for radiative decay rather than non-radiative decay. Considerable work in this area remains.
Early examples of organic electroluminescence were reported by Pope et al. in 1963 [Pope, M.; Kallmann, H.; Magnante, P. J. Chem. Phys. 1962, 38, 2042] who demonstrated blue light emission from single crystal anthracene using very high voltages, ≈400 V. 
Advances on OLED processing over the next two decades were limited to forming thin, light emitting films of organic compounds by vacuum deposition techniques, [Vincett, P. S.; Barlow, W. A.; Hann, R. A.; Roberts, G. G. Thin Solid Films 1982, 94, 476] and lowering driving voltages to  less than 30 V, however these single-layer devices suffered from both poor lifetimes and luminescence efficiencies. In 1987, Tang and Van Slyke [Tang, C. W.; Van Slyke, S. A. Appl. Phys. Lett. 1987, 51, 913] at Eastman Kodak discovered how to make two-layer electroluminescent devices. As shown in FIG. 1, the OLED device 10 was prepared by sandwiching organic hole transport (HT) material 12 and emissive (EM) material 14 between an indium-tin-oxide (ITO) anode 16 and magnesium/silver alloy cathode 18 layers. A conventional electric potential source 20 was connected to the cathode 18 and anode 16. A glass substrate 22 allowed light emission as shown by Arrow 24. The hole transport (HT) and emissive (EM) materials used by Tang and Van Slyke are shown below. 
The key to device performance was the layered architecture sequence: cathode/emissive-electron transport/hole transport/anode. These devices demonstrated brightness, efficiencies, and lifetimes far exceeding anything reported at that time. The materials shown in FIG. 1 were deposited onto indium tin oxide (ITO) coated glass by a vacuum sublimation process to a thickness of ≈25 nm.
In 1990 Burroughs et al. [Burroughs, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackay, K.; Friend, R. H.; Burn, P. L. Nature 1990, 347, 539] developed polymeric OLED devices or PLEDs. In 1992, Braun et al. [Braun, D.; Gutafson, D.; McBranch, D.; Heeger, A., J. J. Appl. Phys., 1992, 72, 546] discovered that poly(p-phenylenevinylene) (PPV), and its derivatives will electroluminesce both green and red light when confined between ITO and aluminum electrodes. 
This work was important because PPV polymers can be deposited by a spin coating process that is more cost effective than vacuum sublimation. Spin coating also facilitates coating larger areas. As a result of these pioneering examples, hundreds of OLED and PLED based papers have been reported by research groups around the world using the following two common materials deposition approaches:
1. Vacuum sublimation of molecular species; and
2. Dip, spin, and spray coating or printing of oligomeric or polymeric materials.
Each method has advantages and disadvantages as outlined below.
Vacuum sublimation works well only with relatively low molecular weight (MW) compounds ( less than 300 g/mol). Such compounds must be purified by sublimation or column chromatography to purities  greater than 99.99% prior to deposition to obtain superior light emitting efficiencies and device lifetimes. Vacuum sublimation allows for multi-layer configurations and very precise control of film thickness, both of which are advantageous in OLED processing. Drawbacks to vacuum sublimation are that it requires very costly equipment and it is limited to deposition on surface areas that are much smaller than surfaces that can be coated using spin coating. Additionally, device performance is adversely affected by the tendency of some sublimed compounds to crystallize with time. To prevent premature crystallization, compounds are currently being designed with high glass transition temperatures (Tgs) and substituents that minimize or prevent crystallization.
Dip coating, spin coating, spray coating, and printing techniques are generally applicable to the deposition of oligomeric and polymeric materials. It permits precise film thickness control, large area coverage and is relatively inexpensive compared to vacuum sublimation. Multi-layer configurations are only possible if the deposited layers are designed with curable functional groups for subsequent cross-linking, or with differing solubilities to prevent re-dissolution during additional coatings. For example, current OLED polymer technology uses a water soluble prepolymer PPV (shown below), that is thermally cured after deposition rendering it insoluble. [Li, X. C.; Moratti, S. C. Semiconducting Polymers as Light-Emitting Materials; Wise, D. L., Wnek, G. E., Trantolo, D. J., Cooper, T. M. and Gresser, J. D., Ed., 1998]. 
Initial luminescent properties for OLEDs based on polymers were often inferior to their molecular counterparts. This was partly due to the difficulty in obtaining high purity material (99.99%) (polydispersity, endgroups, residual solvents and byproducts such as HCl, catalysts, etc.) necessary for efficient devices. However recent studies have shown that carefully purified polymers have similar or even better resultant properties to their molecular counterparts.
Some important parameters to consider when designing novel materials for practical devices include:
1) Methods of depositionxe2x80x94Spin, spray, dip coating and ink (bubble) jet printing are typically more cost effective than vacuum sublimation. Therefore, the luminescent material is preferably usable with spraying, spin coating, dip coating, and/or printing methods.
2) Materials molecular architecturexe2x80x94materials should be designed to prevent or minimize crystallization and/or aggregation that are known to yield inferior device properties.
3) Color tuning and color purityxe2x80x94Luminescent materials should be designed to provide red, green, and blue (RGB) electroluminescence for full color devices. They should be readily purified to xe2x89xa799.99% purity.
4) Increase in device efficiency, brightness, and lifetimexe2x80x94To provide devices for commercial application, materials should provide  greater than 2% external quantum efficiency (2 photons emitted per 100 injected electrons),  greater than 500 cd/m2 operating at  less than 5 V, and luminescence half-lives  greater than 10,000 hours (roughly equivalent to 10 h/day, 6 days/week for 3 years).
5) Construction of efficient device architecturesxe2x80x94The most commonly reported design is shown in FIG. 1. Cathodes are generally prepared by vacuum deposition of Ag/Mg, Ca, or Al. Typically cathodes with lower work functions provide better initial device performancexe2x80x94i.e. Ca less than Al/Li/Ag/Mg less than Al. However, as Ca, Ag, and Mg are more susceptible to oxidation, Al is generally the cathode material of choice. The anodes are typically commercially available ITO coated glass. Studies show that final device performance is directly correlated to the ITO surface properties; thus, extreme care should be taken when selecting the ITO anode. Alternatively, poly(aniline) (PANI), and poly(2,3-ethylenedioxy)thiophene (PEDOT) have been used as anode material on both ITO deposited on glass and flexible Mylar(copyright) substrates.
From the foregoing, it will be appreciated that there is a need in the art for OLED materials that can be easily and highly purified, that have high Tgs and little tendency to crystallize or aggregate, that can be processed by spin coating, dip coating, or printing methods, that are exceptionally resistant to thermal, oxidative, hydrolytic and electrolytic degradation, that can be readily modified to permit tailoring of properties, e.g. stability, electroluminescent efficiencies, solubility, etc., that have low turn-on voltages and relative ease in color tuning.
The present invention includes a method of synthesizing and characterizing organic-inorganic HLED materials based on silsesquioxane architectures. These hybrid materials have the potential to combine the advantages of both molecular and polymer approaches to OLED applications. The silsesquioxane compounds incorporate at least one, and preferably multiple, functional moiety substituents within a single compound. The functional substituents are preferably selected from hole transport (HT), electron transport (ET), and emissive material (EM) moieties and combinations thereof. This offers considerable potential to optimize OLED properties, e.g. luminescence efficiency, brilliance, turn-on voltage, longevity, high density of HT, ET, and EM moieties, etc.
The organic-inorganic HLED silsesquioxane material has the general formula (RSiO1.5)n wherein n typically ranges from 6 to 12 for discrete cage structures, but n can be much larger (up to about 100) in extended polymeric silsesquioxane structures, and R is selected from a plethora of functional groups including hole transport, electron transport, and emissive materials. The hybrids may be synthesized from a series of core polyhedral silsesquioxane intermediates having a variety of reactive substituent groups capable of reacting with functional groups having HT, ET, or EM properties. Examples of typical reactive substituent groups of silsesquioxane intermediates include, but are not limited to, hydrogen, vinyl, phenyl, substituted phenyl, stilbenyl, and substituted stilbenyl, where the phenyl substituents include, halo, amino, hydroxyl, vinyl, unsaturated alkyl, haloalkyl, silyl, etc. Some hybrids may also be synthesized using a direct sol-gel synthesis with Rxe2x80x94Si(OCH2CH3)3 as a starting material.
One organic-inorganic HLED material having an octahedral silsesquioxane structure has the following general structure: 
wherein R1, R2, R3, R4, R5, R6, R7, and R8 are selected from hole transport moieties, electron transport moieties, emissive material moieties, and curable groups including, but not limited to, epoxy, methacrylate, stryl, vinyl, propargyl and combinations thereof. The hybrid material preferably includes at least one HT, ET, or EM moiety. The curable groups are included to facilitate fabrication of multilayer devices, but would not necessarily be needed in a single layer device.
The present invention includes organic-inorganic HLED devices fabricated with an HLED material described above. Such devices typically include an anode containing a high work function metal, metal alloy, or metal oxide, a cathode containing a low work function metal or metal alloy, and a layer of the organic-inorganic luminescent material based upon the silsesquioxane structures described above, physically and thus electrically connected to the anode and cathode. The organic-inorganic HLED devices are preferably fabricated with a transparent substrate such as glass or clear plastic. The anode is preferably selected from gold, silver, copper, indium-tin oxide (ITO), fluorine-tin oxide (FTO), or other transparent conducting oxide or polymeric materials. Below about 50 nm, gold, silver, and copper layers are semi-transparent rendering them useful anode materials. The cathode is selected from calcium, magnesium, lithium, sodium, aluminum, and alloys thereof. The cathode may also be selected from mixtures of calcium, magnesium, lithium, sodium, and aluminum with halogen salts of group IA metals (Li, Na, K, Rb, Cs). For example, a commercially available Al/Li alloy (98.5% Al/1.5% Li) Al is relatively stable in air and has a work function similar to Ca. Similarly, LiF, CsF, and other salts can lower the work function when deposited at the Al/organic interface.
The organic-inorganic HLED device can include multiple layers of organic-inorganic luminescent materials. Each layer can be based upon luminescent material having different functional moiety substituents selected from hole transport, electron transport, and emissive material moieties. For instance, the HLED device may include a layer having hole transport substituents, a layer having electron transport substituents with the hole and/or electron transport layer(s) also serving as the emissive component. Other substituents can be included to provide desired functional or physical characteristics, such as enhanced hydrophobicity, adhesive character, and dyes for color tuning to the HLED material. The HLED materials can also be designed to include multiple properties, such as balanced electron and hole transport as well as emissive properties.
The organic-inorganic HLED device can be fabricated with a single layer of the organic-inorganic luminescent material that contains hole transport, electron transport, and emissive material substituent moieties on a silsesquioxane structure. HLED devices can also be fabricated with multiple layers containing emissive, hole transport, and electron transport layers balanced to provide the desired luminescent properties.